I do think the pro controller is a viable option, especially if you're new to the series or just didn't really ever play the Gamecube.
However, if you are a long-time fan of the series, you probably already understand the love for the controller. At the very least, you could save up your money to get the two of them, provided they don't go out of circulation. Still, if you don't think it's worth it, it probably isn't. As was mentioned, it's kinda expensive to double the cost of one game and only be able to use that cost towards the one game.
I have to say, though, the new SSB controller's button's are stickier than I remember. I bought the white Japanese controller online the other day, and having played with both of them back to back, and both of them being brand-spanking new, I like the feel of the white one better. If you are going to get a GC controller, in my opinion, your best option is not the new SSB controller. I picked one up for the novelty of it, though. I mean, it's Smash Bros., right? I have disposable income. Why not use it? 'MURICA.